Striated membranous structures which consisted of sheets or ribbons of 130 to 220 A in thickness and showed variable patterns of periodic substructure were found in extracellular locations in atrioventricular valves, myocardium and endocardium of six patients with various types of heart disease. These structures may result from an unusual pattern of arrangement of basement membrane material. BIBLIOGRAPHIC REFERENCES: Renteria, V.G., Ferrans, V.J., and Jones, M.: Striated membranous structures in human hearts. An ultrastructural study. Am J Pathol 85: 85-98, 1976.